As Sound Transit marks its 20th birthday, it faces the biggest-ever threat to its future in the form of an emerging transportation alternative that may well cause voters in the three Central Puget Sound counties to reject the agency's $54 billion transportation package to allow the alternative time to develop.

Not even in their darkest nightmare would Sound Transit's board and the proponents of its megabillion-dollar ballot measure likely have envisioned the emergence of a growing fervor over a new transportation innovation just as the time for a November voter decision on dramatically extending the rail-based package nears.

The transportation innovation that's attracting increasing attention is autonomous vehicles, previously referred to as self-driving cars, with both automobile and truck manufacturers projecting emergence of fully autonomous vehicles within five years. And the Seattle area is being talked up as the nation's launch region for this development because companies like Google, Car2Go and ReachNow have committed to bring that about.

The challenge facing Sound Transit is that its proposal would put a lock on the region's transportation future for the next quarter century, tying it to a system for which rail is the keystone. By then autonomous vehicles and the congestion-easing result of their emergence might well render rail the transportation innovation of yesterday.

And the uncertainty surrounding the transportation future has created a growing sense, expressed not just by Sound Transit critics but also some longtime supporters, that rather than a full-blown package committing the region to a 25-year plan, a series of packages should be placed before the voters. The most recent example of growing concern over the measure called ST-3 was the Bellevue Chamber of Commerce board's decision Tuesday to oppose it.

Those pressing the idea of sending Sound Transit back to the drawing board would seek ballot proposals in staged packages, with a vote to provide funding for one segment, which would be followed by another vote when that project was completed, and so on. Then at any point, the voters could decide times have indeed changed and no more Sound Transit rail construction is desired.

As one of those longtime supporter put it when I called to get his candid thoughts: "If you are saying the voters should be offered segments of the total plan over a period of years as each prior segment is completed, of course that's logical."

But Sound Transit, officially the Central Puget Sound Regional Transit Authority, formed in 1996 by the county councils of King, Pierce, and Snohomish Counties, is looking to corral all 25 years' worth of funding from voters. There is a clear Sound Transit reluctance to even contemplate going back to the drawing board.

As longtime Sound Transit critic, Bellevue developer and business leader Kemper Freeman Jr., sees it, Sound Transit realizes that ST-3 is likely the last time voters might be willing to consider a mega transportation package with taxes that will hit every property owner in the three counties. Too many things, including transportation alternatives and other uses for that massive property tax amount, are certain to emerge in future years.

Intriguingly, this is the second time in its 20 years that an alternative to Sound Transit's rail focus has been offered. Despite the business and political credentials of the five people who teamed up, a year after Sound Transit began operation, to suggest a lower-cost and more efficient idea than the then-planned $1.6 billion Link Light Rail, the idea was basically brushed aside back in 2000.

The plan was called Ride Free Express, offered by two former governors John Spellman, a Republican, and Democrat Booth Gardner, along with John Runstad and Matt Griffin, two well-regarded business leaders, and Charles Collins, one of the region's long-respected transportation experts.

The plan would have eliminated fares for existing as well an expanded express bus fleet and created vanpools, reducing peak congestion by 5 percent at a price a sixth of the cost of new riders on Sound Transit's Link Light Rail, "even assuming they could build, LINK for the original $1.6 billion," Collins said. A recent Seattle Times analysis showed that in the end LINK wasn't built for that price, actually exceeding its budget by 87 percent.

"All of our projections, including that our plan would attract six times the number of new riders, flowed from well-established and independent market studies or actual transit experience," Collins notes. "Not a single board member except Rob McKenna thought that the issues we raised were even slightly interesting."

"They were committed to a project whereas we wanted to reduce congestion," Collins summarized pointly.

"Nothing has changed," said Collins, whose credentials include having been Spellman's Chief King County Adminstrator, Director of Metro Transit and chair of the Northwest Power Planning Council, the State Higher Education Coordinating Board and the State Commission on Student Learning.

Indeed while Sound Transit operates express bus services in addition to rail and light rail service to the region, there has been little doubt in the community that members of the board view themselves as creators of the region's light rail system.

Sound Transit and its proponents have routinely tried to picture the opposition as primarily Kemper Freeman., since a wealthy Eastside businessman makes an easy target for those Seattlites who view rail as something approaching Holy Grail.

Collins, with impeccable credentials for public service, business success and transportation expertise, as well as being a decorated Vietnam veteran and retired Army Reserve Brigadier General, makes an opponent who many Sound Transit believers will find it uncomfortable to attack.

"If we are committed for 25 years and a good idea like autonomous van pools takes shape, good luck since the bond attorneys have made sure the money can't be diverted," Collins told me. "And autonomous van pools would be a good idea and could also be an energy answer."

Freeman sought this year to boost his years-long campaign for roads over rails with report he funded called Mobility 21 that outlined a fact-based alternative to the existing long-range plans. He has presented Mobility 21 at an array of speaking engagements around the region.

Freeman told me the first presentation on the Mobility 21 study was made to officials of the Puget Sound Regional Council, which oversees dispensing federal dollars to the four counties.

"They admitted to us that the idea of autonomous cars had never been envisioned in their 25-year plan," freeman said.

Will autonomous vehicles become an ubiquitous presence on the region's roadways soon? Of course not. But technological advancements, including accident-avoidance devices, in vehicles before that happens will enhance congestion-reductions efforts. And some such technological advances could require commitment of dollars from the public, which would be more difficult to draw out if $54 billion in taxes is still being imposed.

And it's interesting that Daimler Trucks North America CEO Martin Daum talked recently about how he allowed a robotic truck to drive him nearly 25 miles, without his ever touching the steering wheel or brakes. He said his digital pilot used a combination of GPS, map data and sensors to drive the autonomous truck across highways and two-way streets.

And Freeman admitted to me, in one interview, that he has taken a half dozen trips, logging up to 125 miles, both freeway and city streets, with his autonomous Tesla. He said his hands were poised beneath the steering wheel in case his intervention was needed, bur that he never actually had his hands on the wheel.

Freeman and other ST-3 opponents haven't yet been seeking slogans for the final months of their campaign, but given the new realities facing the $54 billion plan, it could be referred at this time as "not a sound plan."

Bellevue developer Kemper Freeman Jr.'s years-long campaign for roads over rails as the way to address the region's transportation needs may be getting a boost just as he is releasing and beginning to promote a report called Mobility 21 as an alternative to the existing long-range plans.

Having completed the Mobility 21 document, for which his Kemper Development Co. was key funder, he is now seeking to generate renewed discussion among policymakers and business leaders in Seattle and on the Eastside about the region's transportation future.

An unexpected assist for the highways believers was the announcement this week from Gov. Jay Inslee that he will be proposing additional lanes on I-405 in the hope of alleviating some of the congestion on the Eastside freeway that has grown dramatically worse of late, and ease the anger of Eastside motorist about it.

Freeman hopes the governor's announcement, which he applauded, may be the first indication of a broad-based effort to force a rethinking of already approved regional transportation plans that focus on light- rail as a key component of long-term plans rather than solutions needed now that focus on roads.

The more long-term boost, by which he hopes he can bring a new emphasis to his argument that more of projected funding should go to roadway systems, may well be in focusing on the greater efficiency to be derived from evolving technology for highways and vehicles.

An underpinning of Freeman's hope to create discussion on new transportation thinking is his focus on the importance of Seattle as the "Super Regional Center," and the importance or access to it for the 8- to 10-million people in the region, including the 670,000 for which Bellevue is a sub-regional center.

"What Bellevue and Seattle have in common is we are both driven by populations far bigger than our immediate city limits," he added, noting that a key roles for the Super Regional and Sub Regional cities is ensuring access.

But he makes clear he views the challenges to a successful synergy between Seattle and Bellevue relate to what he has long viewed as misguided transportation planning for the region.

And he cautions that he has a concern that another pitfall might be what he perceives as the inability of the leadership of Seattle, as that Super Regional Center, to understand that the impact of their decisions go far beyond the city's 600,000 population. And that they have some responsibility to consider those broader impacts on the region.

"Seattle scares me because the rest of us in the region need them to be the Super Regional City and I don't get the impression they are trying to do that," Freeman said. "Our premise is that Seattle and Bellevue each has a role to play in the regional picture and Seattle is not playing its role."

And in discussing the study, Freeman, a generally soft-spoken businessman, raises his voice in anger as he suggests Seattle and the Eastside have a common enemy, Sound Transit. It's Sound Transit's focus on rail as a keystone in the region's transportation future that Freeman has fought for years, including his lawsuit to stop construction of a light-rail line to the Eastside across I-90 that was rejected by the State Supreme Court in the fall of 2013.

Freeman is a believer in rapid transit as a part of the solution, but bus rapid transit (generally referred to by planners around the county as BRT), with center lanes of the I-90 freeway dedicated to buses rather than to rail, contending that when buses reach the Eastside, or suburban points north or south, they can carry passengers to more stops with greater flexibility than light-rail.

And he emphasizes, at a time of increasing frustration about the regions gridlock and congestion, that the BRT approach can begin service and help bring traffic relief in several years rather than decades and at dramatic lower costs.

Now comes what he sees as a boost to discussions that he hopes will lead to a revisit not just the I-90 rail plan but a need to rethink exiting plans.

Freeman, owner of Bellevue Square, Bellevue Place and Lincoln Square as well as emerging pieces of what his marketing folks refer to as The Bellevue Collection, a 6-million-square-foot portfolio of "commerce, style and culture," explained to Eastside business leaders recently about the rationale for Mobility 21.

The point of having produced Mobility 21 at this time is based on what the project describes as "Five Critical Realities," the first of which, that congestion is worsening, is an obvious reality that Freeman hopes may create some new converts in business and government to his goal of greater spending on the "roadway system."

But the more long-term boost, he hopes, he can bring a new emphasis to his argument that more of projected funding should go to those roadway systems by focusing on the greater efficiency to be derived from evolving technology for highways and vehicles.

Specifically, Mobility 21 suggests that automated driver assistance systems and collision-preventing features like adaptive cruise control, automated lane keeping on freeways, radar breaking and blind-spot monitoring will lead to 50 percent more capacity per freeway lane.

In fact, as an observer rather than an advocate for either position, I'm struck by the fact a massive worldwide effort is underway to radically change the shape of personal mobility with smaller, lighter, cleaner, collision-proof vehicles running on existing roadways.

There's an inescapable sense that those advancements will require transportation planners to weigh anew whether the transportation-expenditure priorities should remain the same or be re-evaluated.

And any decision on re-evaluating and possibly revising transportation plans to reflect emerging personal mobility technologies needs to be done with only transportation benefits as the goal, with no consideration for the politics of what kind of transportation some community or business groups might wish to emphasize.